The history of zeolites began with the discovery of stilbite in 1756 by the Swedish mineralogist A. Cronsted. Zeolite means “boiling stone” and refers to the frothy mass which can result when a zeolite is fused in a blowpipe. Volatile zeolitic water forms bubbles within the melt.
Zeolites are crystalline aluminosilicates having as a fundamental unit a tetrahedral complex consisting of Si4+ and Al3+ in tetrahedral coordination with four oxygens. Those tetrahedral units of [SiO4] and [AlO4]− are linked to each other by shared oxygens and in this way they form three-dimensional networks. The building of such networks produces channels and cavities of molecular dimensions. Water molecules and charged compensating cations are found inside the channels and cavities of the zeolitic networks.
Even though there was much knowledge about zeolites and its properties, it was until the middle of this century that commercial preparation and use of zeolites was possible. This advance allowed more research into the synthesis and modification of zeolitic materials.
The modification of the physical-chemical properties of zeolitic molecular sieve by the incorporation of other elements different from silicon and aluminum can be achieved through one of the following ways:
1.—Incorporation through ion exchange
2.—Incorporation through impregnation
3.—Incorporation into the synthesis gel.
The most common and well known form of introducing different elements in the channels and cavities of zeolitic molecular sieves is through ion exchanging. In this way, the compensating cation balancing the negative charge of the framework (usually sodium) is replaced by a new cation after ion exchange is done. In this case, the new cation is located inside the channels and cavities of the zeolite but, it is not coordinated with the silicon atoms throughout the oxygen atoms.
The incorporation of other chemical elements in the zeolitic molecular sieve through impregnation is another common way of modifying the properties of zeolitic materials. For this case, most of the element incorporated in the zeolite is found in the surface of the crystallites of the zeolitic material.
The incorporation into the synthesis gel of other chemical elements to produce zeolitic molecular sieves allowed an important advance in this area of research. This variation not only has modified the physical-chemical properties of the zeolitic materials of known structures, but also has given rise to the production of new structures unknown in the aluminosilicate frameworks.
Patent and open literature have shown two important groups of zeolitic molecular sieve which incorporate other elements besides silicon and aluminum. These two main groups are the metallosilicates and the metalloaluminosphosphates. The metallosilicates are molecular sieves in which the aluminum is replaced by another element like gallium, iron, boron, titanium, zinc, etc. The metalloaluminophosphates are molecular sieves in which the aluminophosphate framework is modified by the incorporation of another element like magnesium, iron, cobalt, zinc, etc.
Because the present invention is more related to metallosilicates than to metalloaluminophosphates, the metallosilicates are discussed in more detail. To choose an element to be incorporated into the molecular sieve framework, researchers take into account the possibility that the chosen element can attain tetrahedral coordination as well as the ionic ratio radius of such element. Table 1 shows the elements that can attain a tetrahedral coordination as well as the ionic ratio radius of such elements.
Some of the elements indicated in Table 1 have been claimed to be incorporated into molecular sieve structures of the metallosilicate type. Some examples are: Ironsilicates or Ferrisilicates [U.S. Pat. Nos. 5,013,537; 5,077,026; 4,705,675; 4,851,602; 4,868,146 and 4,564,511], zincosilicates [U.S. Pat. Nos. 5,137,706; 4,670,617; 4,962,266; 4,329,328; 3,941,871 and 4,329,328], gallosilicates [U.S. Pat. Nos. 5,354,719; 5,365,002; 4,585,641; 5,064,793; 5,409,685; 4,968,650; 5,158,757; 5,133,951; 5,273,737; 5,466,432 and 5,035,868], zirconosilicates [Rakshe et al, Journal of Catalysis, 163: 501-505, 1996; Rakshe et al, Catalysis Letters, 45: 41-50, 1997; U.S. Pat. Nos. 4,935,561 and 5,338,527], chromosilicates [U.S. Pat. Nos. 4,299,808; 4,405,502; 4,431,748; 4,363,718; and 4,4534,365], magnesosilicates [U.S. Pat. Nos. 4,623,530 and 4,732,747] and titanosilicates [U.S. Pat. Nos. 5,466,835; 5,374,747; 4,827,068; 5,354,875 and 4,828,812].
TABLE 1Metal ions that can attain tetrahedral coordinationand their ionic crystal radii.Metal ionRadius (Å)Al3+0.530As5+0.475B3+0.250Be2+0.410Co2+0.720Cr4+0.550Cr5+0.485Fe2+0.770Fe3+0.630Ga3+0.610Ge4+0.530Hf4+0.720In3+0.760Mg2+0.710Mn2+0.800Mn4+0.530Mn5+0.470Mn6+0.395Ni2+0.620P5+0.310Si4+0.400Sn4+0.690Ti4+0.560V5+0.495Zn2+0.740Zr4+0.730
The conventional preparation of metallosilicates succeeds only if organic structure guiding compounds (“organic templates”) are added to the synthesis mixture. In general, tetraalkylamonium compounds, tertiary and secondary amines, alcohols, ethers, and heterocyclic compounds are used as organic templates.
All these known methods of producing metallosilicates have a series of serious disadvantages if it is desired to produce them in a commercial scale. For instance, those organic templates used are toxic and easily flammable so, since the synthesis must be carried out under hydrothermal conditions and a high pressure in autoclaves, an escape of these templates into the atmosphere can never be completely prevented. Also, the use of templates increases the cost of production of the material because the template is expensive and because the effluent from the production of the metallosilicate also contains toxic materials which require expensive and careful disposal in order to prevent contamination of the environment.
Adding to this, the metallosilicate obtained has organic material inside the channels and cavities so, to be useful as a catalysts or adsorbent, this organic material must be removed from the lattice. The removal of the organic template is carried out by combustion at high temperatures. The removal of the template can cause damage to the lattice structure of the metallosilicate molecular sieve and thus diminish its catalytic and adsorption properties.
The metalloaluminosilicate is another group of zeolitic molecular sieves that can be prepared, however, research in this area is not as popular as it is with the metalloaluminophosphates and metallosilicates. In spite of that, in the patent literature it is possible to find some examples of this type of materials. The preparation of iron- titano- and galloaluminosilicates can be found in U.S. Pat. No. 5,176,817; U.S. Pat. No. 5,098,687, U.S. Pat. No. 4,892,720; U.S. Pat. No. 5,233,097; U.S. Pat. No. 4,804,647; and U.S. Pat. No. 5,057,203. For those cases, the preparation of the material is by a post synthesis treatment. An aluminosilicate zeolite is put in contact with a slurry of a fluoro salt of titanium or/and iron or a gallium salt and then some of the aluminum is replaced by titanium, iron or gallium. This methodology has some disadvantages because of the extra steps required to produce the material.
The ideal thing to do would be to add the desired element into the synthesis gel and then through a hydrothermal process get the metalloaluminosilicate material. In the patent literature is possible to find some examples of this type of procedure. U.S. Pat. No. 5,648,558 teaches the preparation and use of metalloaluminosilicates of the BEA topology with chromium, zinc, iron, cobalt, gallium, tin, nickel, lead, indium, copper and boron. U.S. Pat. No. 4,670,474 teaches the preparation of ferrimetallosilicates with aluminum, titanium, and manganese. U.S. Pat. No. 4,994,250 teaches the preparation of a galloaluminosilicate material having the OFF topology. U.S. Pat. Nos. 4,761,511; 5,456,822; 5,281,566; 5,336,393; 4,994,254 teach the preparation of galloaluminosilicates of the MFI topology. U.S. Pat. No. 5,354,719 teaches the preparation of metalloaluminosilicates of the MFI topology with gallium and chromium. These examples of metalloaluminosilicates require the use of organic templates or seeding procedures so, these methods of preparation of metalloaluminosilicates have similar problems to those described above for metallosilicate methods of preparation.